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Hybrid Constructed Wetland

Author/Compiled by

Beat Stauffer (seecon international gmbh)

Dorothee Spuhler (seecon international gmbh)

Executive Summary

Different types of constructed wetlands can be combined in order to achieve a higher treatment efficiency by using the advantages of individual systems. Most hybrid constructed wetlands combine vertical filter and horizontal filter stages. The vertical- horizontal filter system was originally designed in the late 1950s and the early 1960s but the use of hybrid systems was very limited. In the 1980s hybrid constructed wetlands were built in France and United Kingdom. At present, hybrid constructed wetlands are in operation in many countries around the world. They need expert design, but they can be built mostly with locally available material and the community can be trained for operation and maintenance. The effluent can be used for e.g. irrigation and aquaculture or safely be discharged to receiving water bodies.

Advantages

In general they have a higher treatment effect than single systems, especially for nitrogen, because the advantages of the different filters can be combined

As long as no free-surface filter is involved, they do not lead to increased mosquito breeding

These three types of CWs may be combined with each other in hybrid constructed wetlands in order to exploit the specific advantages of the different systems.

One of the main advantages of CWs is that they are natural systems and thus not require chemicals, energy or high-tech infrastructure. Moreover, they are suited to be combined with aquaculture or sustainable agriculture (irrigation).

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Treatment Process and Basic Design Principles

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Almost every type of constructed wetlands can be combined to fit different types of wastewater. However, hybrid systems are comprised most frequently of vertical flow and horizontal flow systems arranged in a staged manner. Horizontal flow systems cannot provide nitrification because of their limited oxygen transfer capacity. Vertical flow systems, on the other hand, do provide good conditions for nitrification but denitrification does not really occur in these systems. In hybrid systems (also sometimes called combined constructed wetland) the advantages of the horizontal and vertical flow systems can be combined to complement processes in each system to produce an effluent low in BOD, which is fully nitrified and partly denitrified and hence has much lower total-N outflow concentrations (VYMAZAL 2005). At present, hybrid constructed wetlands are in operation in many countries around the world and they are used especially when removal of ammonia-N and total-N is required (VYMAZAL 2010).

VF-HF hybrid constructed wetland based on the concept of Seidel. Source: VYMAZAL (2005)

Many of these hybrid systems are based on the original hybrid systems developed by Seidel at the Max Planck Institute (Germany). The design consists of two stages of several parallel vertical flow beds (usually planted with Phragmites australis), followed by two or three horizontal beds (planted e.g. Typha or Carex). In these systems, the vertical flow beds are loaded with pre-treated wastewater for 1–2 days, and then allowed to dry out for 4–8 days. The thin crust of solids that forms on top of the vertical flow beds is mineralised during the rest period (VYMAZAL and KRÖPFELOVÀ 2011).

In the mid-1990s, Johansen and Brix introduced a horizontal-vertical flow hybrid system with a large horizontal flow bed placed first and a small vertical flow bed as the second stage. In this system nitrification takes place in the vertical flow stage at the end of the process sequence. If nitrate removal is needed, it is necessary to pump the effluent back to the front end of the system where denitrification can take place in the less aerobic horizontal flow bed using the raw feed as a source of carbon needed for denitrification (VYMAZAL and KRÖPFELOVÀ 2011).

Hybrid systems are more expensive than non-hybrid systems (free surface-flow constructed wetlands, horizontal and vertical flow constructed wetlands). A larger area is required and they are more complicated and complex to operate, especially for adjustment and monitoring of the loads (HOFFMANN et al. 2010). However, compared to other intensive high-rate aerobic treatment options (e.g. activated sludge), constructed wetlands are natural systems, which work extensively. That means treatment may require more land and time, but you can safe costs because there is no or just a low energy need. Furthermore, there is no need for sophisticated equipment, expensive spare parts or chemicals (GAUSS 2008).

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Operation and Maintenance

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Basically, O&M is similar to horizontal flow constructed wetlands or vertical flow constructed wetlands. The difficulty is to ensure that the system is adjusted and the filters loaded correctly. That means it needs expert knowledge and skilled operators. It should be considered that a wetland system will require maintenance for the entire duration of its life.

The constructed wetland in the peri-urban area of Bayawan, Philippines, is designed for 700 people. Pre-treated wastewater is pumped into the header tanks; from there it flows by gravity to the vertical filer and later to the horizontal filter. Source: LIPKOW and MUENCH (2010)

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Health Aspects

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The risk of mosquito breeding in warm countries is low as long as there is no open water surface (e.g. free surface constructed wetlands). It should be ensured that residents do not come in contact with the sludge/wastewater in the pre-treatment facility nor with the influent of the filter because of the risk if infection (TILLEY et al. 2008)

The effluents can generally meet the standards for pathogen levels for safe discharge to the environment without further treatment. In case of domestic wastewater, the situation could be different and for safety reasons disinfection (by tertiary treatment) might be necessary, depending on the intended reuse application (HOFFMANN et al. 2010).

The biggest health risk arises from settled wastewater in the pre-treatment facility; this should be considered during inspections and emptying. A proper emptying process (human powered or motorised) can decrease the health risks (TILLEY et al. 2008). After that, also sludge must be treated correctly, for example in drying beds or composting facilities.

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At a Glance

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Working Principle

Any type of constructed wetlands may be combined with each other in order to exploit the specific advantages of the different systems. In general, horizontal and vertical flow filters are combined in series.

Capacity/Adequacy

Hybrid constructed wetlands have a good performance and if land is available they are able to treat wastewater for bigger communities in peri-urban areas (see case study LIPKOW and MUENCH 2010).

Performance

If designed and operated correctly they have very high performance, because the advantages of horizontal and vertical flow filter beds are combined.

Costs

Capital costs are higher than for single constructed wetlands, because the system requires a lot of land. Furthermore, costs depend on the prices of the filter medium (sand or gravel).

Treatment performance is satisfactory as long as the system is operated correctly. Clogging of the filter bed is a main risk of constructed wetlands.

Main strength

Hybrid constructed wetlands combine various types of constructed wetlands in order to achieve higher treatment effect especially for nitrogen removal.

Main weakness

Very large surface area required, as well as extensive construction knowledge and experience, risk of clogging if waste water is not well pre-treated, high quality filter material is not always available and expensive; expertise required for design, construction and monitoring (MOREL and DIENER 2006).

Hybrid constructed wetlands, even though natural (extensive) treatments such as free surface flow, horizontal and vertical flow constructed wetland require pumps and some skilled operation and maintenance to adjust the filter loads correctly. Therefore they are most suited so small communities organised enough to thoroughly plan and maintain the wetland for the duration of its life.

Constructed wetlands are best suited to warm climates but can be designed to tolerate some freezing and periods of low biological activity (TILLEY et al. 2008). Shade from plants and protection from wind mixing is limiting the dissolved oxygen in the water.

Constructed wetlands allow for the combination with aquaculture and agriculture (irrigation) what contributes to the optimisation of the local water and nutrient cycle.

This report provides an overview of how constructed wetlands serve as natural wastewater treatment systems. It focuses especially on the subsurface horizontal flow type—a technology that has high potential for small and medium-size communities because of its simplicity, performance reliability, and low operation and maintenance requirements. The ability of this wetland to reduce pathogens renders the effluent suitable for irrigation of certain crop species if additional health and environmental protection measures are taken. This report describes several experiences with constructed wetland schemes in Central and South America: a full-scale pilot plant in Nicaragua, a community-managed constructed wetland scheme in El Salvador, and other systems in Colombia, Brazil, and Peru.

This publication intends to help spread awareness and knowledge about the technology of subsurface flow constructed wetlands in developing countries. Constructed wetlands (CWs) can be used as part of decentralised wastewater treatment systems, due to their “robust”, “low-tech” nature with none or few moving parts (pumps) and relatively low operational requirements. CWs can be used for the treatment of domestic and municipal wastewater or greywater, and play an important role in many ecological sanitation (ecosan) concepts.

This report compiles international experience in greywater management on household and neighbourhood level in low and middle-income countries. The documented systems, which vary significantly in terms of complexity, performance and costs, range from simple systems for single-house applications (e.g. local infiltration or garden irrigation) to rather complex treatment trains for neighbourhoods (e.g. series of vertical and horizontal-flow planted soil filters).

Hybrid constructed wetland systems have recently been used to treat wastewaters where high demand for removal of ammonia is required. However, these systems have not been used too often for small onsite treatment systems. This study describes an experimental system for mechanically pre-treated wastewater.

This publication intends to help spread awareness and knowledge about the technology of subsurface flow constructed wetlands in developing countries. Constructed wetlands (CWs) can be used as part of decentralised wastewater treatment systems, due to their “robust”, “low-tech” nature with none or few moving parts (pumps) and relatively low operational requirements. CWs can be used for the treatment of domestic and municipal wastewater or greywater, and play an important role in many ecological sanitation (ecosan) concepts.

The underlying philosophy of phytoremediation research at UFZ (Centre for Environmental Research) is to exploit and to optimise the processes in the rhizosphere. Low-cost, simple systems will be developed to control the environmental problems of different countries in several continents irrespective of their industrial capabilities and conditions – without losing sight of the key principle of cleaning up polluted environmental media in a natural, ecologically balanced way.

This report compiles international experience in greywater management on household and neighbourhood level in low and middle-income countries. The documented systems, which vary significantly in terms of complexity, performance and costs, range from simple systems for single-house applications (e.g. local infiltration or garden irrigation) to rather complex treatment trains for neighbourhoods (e.g. series of vertical and horizontal-flow planted soil filters).

Exhaustive report on technological, operational and economic aspects of decentralised waste water treatment systems. Spreadsheet examples support the reader in designing and planning waste water treatment systems components.

This manual has been prepared as a general guide to the design, construction, operation and maintenance of constructed wetlands for the treatment of domestic wastewater as well as introduction to the design of constructed wetland for sludge drying.

These guidance notes are designed to provide state governments and urban local bodies with additional information on available technologies on sanitation. The notes also aid in making an informed choice and explain the suitability of approaches.

WSP
(2008):
Technology Options for Urban Sanitation in India. A Guide to Decision-Making.
pdf presentation.
New Delhi:
Water and Sanitation Program (WSP)
URL
[Accessed: 03.06.2019]

This paper describes the importance of small-scale decentralised wastewater treatment using reed bed treatment systems (constructed wetlands) in Nepal. It shows how public/community participation can support small-scale construction work while ensuring checks on quality and price of construction, including examples.

There is no single design solution to sanitation. But there are universal principles for systematically and safely detoxifying human excreta, without contaminating, wasting or even using water. Ecological sanitation design — which is focused on sustainability through reuse and recycling — offers workable solutions that are gaining footholds around the world, as Nature explores on the following pages through the work of Peter Morgan in Zimbabwe, Ralf Otterpohl and his team in Germany, Shunmuga Paramasivan in India, and Ed Harrington and his colleagues in California.

This report provides an overview of how constructed wetlands serve as natural wastewater treatment systems. It focuses especially on the subsurface horizontal flow type—a technology that has high potential for small and medium-size communities because of its simplicity, performance reliability, and low operation and maintenance requirements. The ability of this wetland to reduce pathogens renders the effluent suitable for irrigation of certain crop species if additional health and environmental protection measures are taken. This report describes several experiences with constructed wetland schemes in Central and South America: a full-scale pilot plant in Nicaragua, a community-managed constructed wetland scheme in El Salvador, and other systems in Colombia, Brazil, and Peru.

Within the BMZ (German Federal Ministry for Economic Cooperation and Development) financed project on “Advice on the Decentralisation of the Water and Sewerage Sector in Albania” the GIZ and MPWT (Albanian Ministry of Public Works and Transport) initiated the pilot constructed wetland to raise awareness for low cost, appropriate and decentralised sanitation technologies in line with EU standards. It is aimed to be used as a model treatment plant by the main actors of the sector for training, demonstration, research and replication in peri-urban and rural areas of Albania.

This document provides brief descriptions of 17 wetland treatment systems from that are providing significant water quality benefits while demonstrating additional benefits such as wildlife habitat. The projects described include systems involving both constructed and natural wetlands, habitat creation and restoration, and the improvement of municipal effluent, urban stormwater and river water quality. Each project description was developed by individuals directly involved with or very familiar with the project in a format that could also be used as a stand-alone brochure or handout for project visitors.

Hybrid constructed wetland systems have recently been used to treat wastewaters where high demand for removal of ammonia is required. However, these systems have not been used too often for small onsite treatment systems. This study describes an experimental system for mechanically pre-treated wastewater.

Despite the production of more food and extraction of more water globally, wetlands continue to decline and public health and living standards for many do not improve. Why is this – and what needs to change to improve the situation? If we manage wetlands better, can we improve the health and well-being of people? Indeed, why is this important? This report seeks to address these questions.